1. Field of the Invention
The present invention relates to a switching power supply apparatus for switching an input direct-current (DC) voltage with a switching element to convert the input DC voltage into an output DC voltage having a value different from the input voltage.
2. Description of the Related Art
Generally, switching power supply apparatuses are known as DC-DC converters for converting an input DC voltage into an output DC voltage having a desired value.
FIG. 1 is a block diagram conceptually illustrating an example of a conventional DC-DC converter. In FIG. 1, a switching element 1 is on/off controlled by a driving signal 3 generated by a switching element driver circuit 2 to switch on or off a DC voltage of a DC power supply 4. A rectangular wave voltage 5 generated by the switching is converted into a DC voltage by a rectifying/smoothing circuit 6 and outputted from output terminals 7, 8. A part of the output voltage is fed back to the driver circuit 2 such that the driver circuit 2 outputs the driving signal 3 in accordance with the output voltage. For the switching element 1, an FET (field effect transistor), a bipolar transistor, or other semiconductor switching elements may be used.
Switching power supply apparatuses as described above have been widely utilized in recent years as power supply circuits for a variety of electronic systems such as information processing systems and so on because of their features of compact size and high efficiency.
FIG. 2 illustrates an example of a specific configuration of the circuit shown in FIG. 1, where portions having the same functions as those in FIG. 1 are designated by the same reference numerals. Also, FIG. 2 illustrates a converter by using an FET as a switching element 1.
In FIG. 2, a driving signal 3 outputted from a driver circuit 2 is applied to a control terminal of the switching element 1 (the gate terminal of the FET) through a capacitor 9, a resistor 10 and a resistor 11 for a stable switching operation. The switching element 1 turns on when the driving signal 3 is at a positive voltage and turns off when the driving signal 3 is at zero voltage.
The on/off operation of the switching element 1 switches a current from a DC power supply 4 through a leak inductance 12 and a primary winding of a transformer 13, whereby a rectangular wave voltage is generated on the secondary side of the transformer 13. The rectangular wave voltage is converted into a DC voltage by a rectifying/smoothing circuit 6 including a diode 14 and a capacitor 15, and then outputted from output terminals 7, 8.
Generally, it is known that when a higher switching frequency is employed in a switching power supply apparatus, components thereof can be reduced in size. However, as the switching frequency becomes higher, the switching power supply apparatus tends to increase power loss in a voltage converting operation to such a degree that the power loss caused by an increase in the switching frequency reaches an amount which cannot be neglected with respect to the volume of the apparatus. Therefore, a switching frequency is restricted by a certain upper limit.
The power loss as mentioned above is predominantly caused by loss due to the overlapping of voltages and currents generated during a certain time period in a state transition of the switching element 1 from ON to OFF and from OFF to ON and loss due to the charging and discharging of a parasitic capacitance of the switching element 1, in addition to loss due to the capacitor and diode constituting the rectifying/smoothing circuit 6. For reducing the power loss when a switching operation is performed, it is necessary to reduce a transition time of the switching element 1 as much as possible. Conventionally, a variety of circuits have been proposed to meet this requirement.
FIG. 3 illustrates an example of such proposed circuits which is disclosed in JP-A-53-132732.
In FIG. 3, a transformer 13 is provided with a tertiary winding 16, one end of which is connected to a control terminal of a switching element 1 through a resistor 17 and a capacitor 18.
According to this configuration, a flyback pulse, which is a negative voltage generated in the transformer 13 simultaneously with the switching element 1 turning OFF, is applied from the tertiary winding 16 to the control terminal of the switching element 1 through the resistor 17 and the capacitor 18, thereby promptly extracting charges accumulated in the parasitic capacitance of the switching element 1. In this way, the switching element 1 can promptly change from an ON state to an OFF state.
However, when the switching transition time is reduced as described above, a current flowing through the switching element 1 abruptly decreases when the switching element 1 turns OFF, so that energy accumulated in a leak inductance 12 and so on concentrates on the parasitic capacitance of the switching element 1 to abruptly generate a large surge voltage between the drain terminal and the source terminal of the switching element (FET) 1. Such a surge voltage, which causes noise, not only affects other circuit portions but also exceeds the break-down voltage of the switching element 1 depending upon the magnitude of accumulated energy, resulting in breakage of the switching element 1 in some cases. Further, since a major portion of the accumulated energy is eventually consumed by the switching element 1 as loss, the temperature of the switching element 1 rises to sometimes lead to breakage of the switching element 1. These problems are particularly significant in low voltage and large current applications even if switching power supply apparatuses have the same power capacity.
To solve the above-mentioned problem, a switching power supply apparatus having a snubber circuit 19 or 20 has been proposed as illustrated in FIG. 4.
In FIG. 4, the snubber circuit 19, composed of a diode 21, a capacitor 22, and a resistor 23, is arranged in parallel with a primary winding of a transformer 13. Additionally, the snubber circuit 20, composed of a diode 24, a capacitor 25, and a resistor 26, is arranged in parallel with the switching element 1.
According to the above configuration, a surge voltage generated at the time the switching element 1 turns OFF is absorbed by the capacitor 22 through the diode 21 and is consumed by the resistor 23 in the snubber circuit 19. Likewise, a surge voltage generated on the drain side of the switching element 1 is absorbed by the capacitor 25 through the diode 24 and is consumed by the resistor 26 in the snubber circuit 20. In this way, the snubber circuits can reduce surge voltages and noise, and moreover protect the switching element 1 from breakage.
However, the circuit illustrated in FIG. 3 requires the tertiary winding 16 as a power supply for extracting accumulated charges in the switching element 1. If the tertiary winding 16 is not built in the switching power supply apparatus, a dedicated power supply must be externally provided, thus causing several problems such as increased power consumption, increased cost of parts, reduced efficiency, and so on.
On the other hand, when the snubber circuits 19, 20 are arranged as illustrated in FIG. 4, although they provide a large energy absorbing effect, the absorbed energy is consumed by the resistors 23, 26, thus causing problems such as a reduced efficiency, increased capacity and a larger cooling device, and so on.
Some switching power supply apparatuses have both a positive voltage output and a negative voltage output. This type of switching power supply apparatuses will hereinafter be called the "bipolar switching power supply apparatus".
FIG. 5 is a block diagram conceptually illustrating an example of a conventional bipolar switching power supply apparatus.
On a positive voltage output side in FIG. 5, a switching element 51 is on/off controlled by a driving signal 53 generated by a switching element control circuit 52 in order to switch a DC voltage from a DC power supply 54. A rectangular wave voltage 55 generated by the switching is converted into a DC positive voltage by a rectifying/smoothing circuit 56 and outputted from output terminals 57, 58. A part of the output voltage is fed back to the switching element control circuit 52 such that the switching element control circuit 52 outputs the driving signal 53 in accordance with the output voltage.
On a negative voltage output side in FIG. 5, a switching element 59 is on/off controlled by a driving signal 61 generated by a switching element control circuit 60 in order to switch the DC voltage from the DC power supply 54. A rectangular wave voltage 62 generated by the switching is converted into a DC negative voltage by a rectifying/smoothing circuit 63 and outputted from terminals 64, 58. A part of the output voltage is fed back to the switching element control circuit 60 such that the control circuit 60 outputs the driving signal 61 in accordance with the output voltage.
FIG. 6 illustrates an example of a specific configuration of the circuit shown in FIG. 5, where parts having the same functions as those in FIG. 5 are designated by the same reference numerals. Also, FIG. 6 illustrates a bipolar switching power supply apparatus which employs bipolar transistors (hereinafter simply called the "transistors") for switching elements 51, 59.
In the illustrated circuit, a single DC power supply 54 is shared by positive and negative voltage outputs, and a buck chopper switching power supply circuit is used for the positive voltage output while a buck-boost chopper switching power supply circuit is used for the negative voltage output.
A rectifying/smoothing circuit 56 is composed of a flywheel diode 65, a choke coil 66, and a capacitor 67, while a rectifying/smoothing circuit 63 is composed of a flywheel diode 69, a choke coil 68, and a capacitor 70.
In FIG. 6, a driving signal 53, outputted from a control circuit 52, is applied to a control terminal of a switching element, i.e., the base terminal of the transistor 51. The transistor 51 turns ON when a forward bias is applied at the base terminal thereof by the driving signal 53, and turns OFF if no bias is applied by the driving signal 53.
On the positive voltage output side, the transistor 51 is switched in response to the driving signal 53 created by the control circuit 52 in order to switch an input voltage from the DC power supply 54 to stabilize an output voltage. When the transistor 51 turns ON, the input voltage supplies energy to the choke coil 66, the capacitor 67, and a load externally connected to the output terminals 57, 58. In this case, the energy is accumulated in the choke coil 66 by a current flowing through the choke coil 66.
Next, when the transistor 51 turns OFF, the energy accumulated in the choke coil 66 is supplied through the flywheel diode 65 to the capacitor 67 and to the load externally connected to the output terminals 57, 58. By the foregoing operations, a target positive voltage output is generated.
On the negative voltage output side, the transistor 59 is switched in response to a driving signal 61 created by a control circuit 60 in order to switch an input voltage from the DC power supply 54 to stabilize an output voltage. When the transistor 59 turns ON, a current flows from the DC power supply 54 through the transistor 59 and the choke coil 68, whereby energy is accumulated in the choke coil 68.
Next, when the transistor 59 turns OFF, the energy accumulated in the choke coil 68 is supplied through the flywheel diode 69 to the capacitor 70 and to a load externally connected to the terminals 64, 58. By the foregoing operations, a target negative voltage output is generated. In this way, the bipolar switching power supply apparatus is configured for generating both positive and negative outputs.
In the bipolar switching power supply apparatus configured according to the prior art as described above, although the DC power supply 54 is shared by the positive and negative voltage output circuits, the positive and negative voltage output sides respectively have their own switching power supply circuits. For this reason, each of the positive and negative voltage output sides must be provided with a relatively expensive control circuit, a switching element, an inductor element, and so on exclusively for its use, so that the cost required by the manufacturing of the associated circuits increases substantially proportional to the number of output circuits. In addition, since the control circuit is also composed of a large number of elements, a plurality of such control circuits, when incorporated in a single switching power supply apparatus, result in a complicated circuit configuration as a whole.
Generally, negative voltage outputs are often used in power supply applications for driving a microcomputer, for communications, and so on, and most of these applications require small output currents. However, the conventional bipolar switching power supply apparatus described above with reference to FIG. 6 is effective when large currents are required on both the positive and negative voltage output sides. Unfortunately, since no appropriate alternative circuit technique is available for providing a small output current on the negative voltage output side, the foregoing conventional bipolar switching power supply apparatus has been obliged to be used even for the small current applications.